NASA's Phoenix Mars Lander aims to not flame out when it descends to the
arctic surface of the red planet in less than two weeks.

The new Martian probe will try to avoid the fate of its crashed
predecessor, NASA?s Mars Polar Lander, when deploying a parachute and braking
rockets to slow its plunge and make a successful three-point landing.

"This is
not a trip to grandma's house,? said Ed Weiler, associate administrator of
NASA?s Science Mission Directorate at the agency?s headquarters in Washington, D.C. ?Putting a spacecraft safely on Mars is hard and risky.?

Phoenix managers refer to the probe?s descent as ?seven minutes of
terror? that will define the future of the spacecraft?s $420-million mission.
The robotic arm-equipped spacecraft is due to land near the Martian north pole
on May 25 to study nearby water ice and determine if the region was once
habitable for primitive life.

"Hopefully
the outcome will be different from the Mars Polar Lander outcome," said
Rob Grover, NASA engineer at the Jet Propulsion Laboratory in Pasadena, Calif.

Mars Polar
Lander (MPL) entered the Martian atmosphere near the planet?s south pole in
1999, but a software glitch caused a premature shutdown of the spacecraft's
engines. It crashed while falling at 50 mph (80 kph) instead of making a soft
landing. NASA has worked since then to ensure Phoenix doesn?t suffer the same fate.

"The
number one cause was the faulty indicator on touchdown sensor," Grover
told SPACE.com,adding that the sensor falsely told the MPL that
it had already landed.

Engineers
have since corrected the software issue and made the overall system more robust
to avoid future errors.

?We feel
like we have adequately tested this vehicle,? Phoenix project manager Barry Goldstein said in a Tuesday mission
briefing, but added that there is always room for the unexpected. ?We fire 26
pyrotechnic events in the last 14 minutes of this vehicle, and every one of
those has to go off as planned?We?re very hopeful for success on the 25th.?

Phoenix
User?s Guide for Mars Arrival

The exact
fate of the lost MPL remains somewhat uncertain because that probe had no way
of communicating with Earth once it entered the Martian atmosphere. That won't
be the case for Phoenix, which has a small crowd of three
Mars orbiters to watch and relay information from the spacecraft throughout
landing.

For
Goldstein, it is the three-second communications gap between Phoenix?s departure from its cruise stage
and the first signals to its relay network that gives him the shivers. If Phoenix fails to land successfully, any
signals just before landing will prove vital in learning its fate, he said.

?Getting
that communications down is the important thing,? Goldstein said. ?That will be
the three seconds that I?m really biting my nails over.?

"This
is the first time for any Mars landing having orbital relay communications for
both landing and being on the surface," Grover said.

Phoenix will descend and land much the same
way that MPL was meant to, plunging into the Martian atmosphere at about 13,000
mph (21,000 kph). That's similar to respective 2004 descents of NASA?s Spirit
and Opportunity rovers, though Phoenix?s arrival would mark first powered
landing on Mars since NASA?s Viking missions of the 1970s

The probe
combines new technology with proven methods for landing, including an
Apollo-era Earth entry software algorithm to guide the spacecraft's early
descent into the Martian atmosphere.

A
Viking-era parachute is designed to open once Phoenix falls within 7.8 miles (12.6 km) above Mars, creating drag
to slow the spacecraft as it screams through the atmosphere at supersonic
speed. The probe?s landing radar should begin giving altitude and velocity of
descent as Phoenix nears the surface, so that the
onboard computer can make any necessary landing adjustments.

"By
the time you get the parachute opening, there can be significant errors in
positioning on order of kilometers," Grover said. "So that's where
radar is critical, because it turns on and gets fresh knowledge of
altitude."

Vertical
Martian lander

Two minutes
after the parachute deployment, Phoenix will have descended to approximately 0.6 miles (1 km) above the
surface. The lander should then jettison its backshell and freefall for half a
second before lighting up its engines.

Nine of the
twelve engines will pulse furiously 10 times per second  an effect Grover
likened to "coming down on a jackhammer." The three non-pulsing
engines should fire steadily to help ensure added stability.

?Just
before touchdown, we actually pirouette the vehicle,? Goldstein said, adding
that the move will aid Phoenix?s vital solar arrays. ?We actually
turn it so we maximize solar exposure.?

Navigators
at JPL can upload fresh orders to Phoenix?s guidance computer up to three hours before landing, in case course
adjustments are required. However, Grover and other NASA engineers will only be
able to stand by and trust in their spacecraft technology once the Mars lander
begins its descent.